Preparation and characterization of multiwall carbon nanotube/polypyrrole coaxial fibrils

Multiwall carbon nanotube (MWNT)/polypyrrole (PPy) fibrils were fabricated by template-free in situ electrochemical deposition of PPy over MWNTs, and characterized by electron microscopy and electrical measurements. Scanning and transmission electron microscopy studies reveal that PPy coating on the surface of nanotube is quite uniform throughout the length, with the possibility of forming unique Y-junctions. Current (I)-voltage (V) characteristics at various temperatures show nonlinearity due to tunneling and hopping contributions to transport across the barriers. AC conductivity measurements (300-4.2 K) show that the onset frequency scales with temperature, and the nanoscale connectivity in MWNT/PPy fibrils decreases with the lowering of temperature. (C) 2011 Elsevier B.V. All rights reserved.

Metal-insulator transition in perovskite oxides: Tunneling experiments

In this paper we have investigated the composition-driven metal-insulator (MI) transitions in two ABO3 classes of perovskite oxides (LaNixCo1-xO3 and NaxTayW1-yO3) in the composition range close to the critical region by using the tunneling technique. Two types of junctions (point-contact and planar) have been used for the investigation covering the temperature range 0.4 K

A Rotor Flux Estimation During Zero and Active Vector Periods Using Current Error Space Vector From a Hysteresis Controller for a Sensorless Vector Control of IM Drive

This paper proposes a sensorless vector control scheme for general-purpose induction motor drives using the current error space phasor-based hysteresis controller. In this paper, a new technique for sensorless operation is developed to estimate rotor voltage and hence rotor flux position using the stator current error during zero-voltage space vectors. It gives a comparable performance with the vector control drive using sensors especially at a very low speed of operation (less than 1 Hz). Since no voltage sensing is made, the dead-time effect and loss of accuracy in voltage sensing at low speed are avoided here, with the inherent advantages of the current error space phasor-based hysteresis controller. However, appropriate device on-state drops are compensated to achieve a steady-state operation up to less than 1 Hz. Moreover, using a parabolic boundary for current error, the switching frequency of the inverter can be maintained constant for the entire operating speed range. Simple sigma L-s estimation is proposed, and the parameter sensitivity of the control scheme to changes in stator resistance, R-s is also investigated in this paper. Extensive experimental results are shown at speeds less than 1 Hz to verify the proposed concept. The same control scheme is further extended from less than 1 Hz to rated 50 Hz six-step operation of the inverter. Here, the magnetic saturation is ignored in the control scheme.

Effect of Constant and Cyclic Current Stressing on the Evolution of Intermetallic Compound Layers

In this paper the effects of constant and cyclic power loads on the evolution of interfacial reaction layers in lead-free solder interconnections are presented. Firstly, the differences in the growth behavior of intermetallic compound (IMC) layers at the cathode and anode sides of the interconnections are rationalized. This is done by considering the changes in the intrinsic fluxes of elements owing to electromigration as well as taking into account the fact that the growth of Cu3Sn and Cu6Sn5 are coupled via interfacial reactions. In this way, better understanding of the effect of electron flux on the growth of each individual layer in the Cu-Sn system can be achieved. Secondly, it is shown that there is a distinct difference between steady-state current stressing (constant current, constant temperature) and power cycling with alternating on- and off-cycle periods (accompanied by a change of temperature). The reasons behind the observed differences are subsequently discussed. Finally, special care is taken to ensure that the current densities are chosen in such a way that there is no risk for even partial melting of the solder interconnections.

Influence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

We investigate the influence of the ferromagnetic layer on the magnetic and transport properties of YBa2Cu3O7-delta in YBa2Cu3O7-delta (YBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers. The temperature dependent dc magnetization study reveals the presence of magnetic anisotropy in YBCO/LSMO bilayer as compared to the pure YBCO layer. The ac susceptibility study on YBCO/LSMO bilayers reveals stronger pinning and the temperature dependent critical current is found to be less prone to temperature. Besides, the current (I) dependent electrical transport studies on YBCO/LSMO exhibit a significant reduction in the superconducting T-c with increase in I and it follows I-2/3 dependence in accord with the pair breaking effect. The higher reduction of superconducting T-c in YBCO/LSMO is believed to be due to the enhanced pair-breaking induced by the spin polarized carriers being injected into the superconductor. (C) 2011 American Institute of Physics. doi: 10.1063/1.3560029]

Synthetic aperture radar: A focus on current problems

Synthetic aperture radar (SAR) is a powerful tool for mapping and remote sensing. The theory and operation of SAR have seen a period of intense activity in recent years. This paper attempts to review some of the more advanced topics studied in connection with modern SAR systems based on digital processing. Following a brief review of the principles involved in the operation of SAR, attention is focussed on special topics such as advanced SAR modelling and focussing techniques, in particular clutterlock and autofocus, Doppler centroid (DC) estimation methods involving seismic migration technique, moving target imaging, bistatic radar imaging, effects of system nonlinearities, etc.

Study of the electrical properties of pulsed laser ablated (Ba0.5Sr0.5)TiO3 thin films

The laser ablated barium strontium titanate (BST) thin films were characterized in terms of composition, structure, microstructure and electrical properties. Films deposited at 300 degrees C under 50 mTorr oxygen pressure and 3 J cm(-2) laser fluence and further annealed at 600 degrees C in flowing oxygen showed a dielectric constant of 467 and a dissipation factor of 0.02. The room-temperature current-voltage characteristics revealed a space charge limited conduction (SCLC) mechanism, though at low fields the effect of the electrodes was predominant. The conduction mechanism was thoroughly-investigated in terms of Schottky emission at low fields, and bulk-limited SCLC at high fields. The change over to the bulk-limited conduction process from the electrode-limited Schottky emission was, attributed to the process of tunneling through the electrode interface at high fields resulting into the lowering of the electrode contact resistance and consequently giving rise to a bulk limited conduction process. The predominance of SCLC mechanism in the films suggests that the bulk properties are only revealed if the depletion width at the electrode interface is thin enough to allow the tunneling process to take place. This condition is only favorable if the him thickness is high or if the doping concentration is high enough. In the present case the film thickness ranged from 0.3 to 0.7 mu m which was suitable to show the transition mentioned above. (C) 1999 Elsevier Science S.A. All rights reserved.

A current error space vector based hysteresis controller with constant switching frequency and simple online boundary computation for VSI fed IM drive

This paper proposes a simple current error space vector based hysteresis controller for two-level inverter fed Induction Motor (IM) drives. This proposed hysteresis controller retains all advantages of conventional current error space vector based hysteresis controllers like fast dynamic response, simple to implement, adjacent voltage vector switching etc. The additional advantage of this proposed hysteresis controller is that it gives a phase voltage frequency spectrum exactly similar to that of a constant switching frequency space vector pulse width modulated (SVPWM) inverter. In this proposed hysteresis controller the boundary is computed online using estimated stator voltages along alpha and beta axes thus completely eliminating look up tables used for obtaining parabolic hysteresis boundary proposed in. The estimation of stator voltage is carried out using current errors along alpha and beta axes and steady state model of induction motor. The proposed scheme is simple and capable of taking inverter upto six step mode operation, if demanded by drive system. The proposed hysteresis controller based inverter fed drive scheme is simulated extensively using SIMULINK toolbox of MATLAB for steady state and transient performance. The experimental verification for steady state performance of the proposed scheme is carried out on a 3.7kW IM.

Alternating current conduction and impedance spectroscopy analysis on pulsed excimer laser ablated (Pb, La)TiO3 thin films

Lanthanum doped lead titanate (PLT) thin films were identified as the most potential candidates for the pyroelectric and memory applications. PLT thin films were deposited on Pt coated Si by excimer laser ablation technique. The polarization behavior of PLT thin films has been studied over a temperature range of 300 K to 550 K. A universal power law relation was brought into picture to explain the frequency dependence of ac conductivity. At higher frequency region ac conductivity of PLT thin films become temperature independent. The temperature dependence of ac conductivity and the relaxation time is analyzed in detail. The activation energy obtained from the ac conductivity was attributed to the shallow trap controlled space charge conduction in the bulk of the sample. The impedance analysis for PLT thin films were also performed to get insight of the microscopic parameters, like grain, grain boundary, and film-electrode interface etc. The imaginary component of impedance Z" exhibited different peak maxima at different temperatures. Different types of mechanisms were analyzed in detail to explain the dielectric relaxation behavior in the PLT thin films.

Carrier density-dependent transport in poly(3-methylthiophene): from injection-limited to space-charge-limited current

Current-voltage (I-V) and impedance measurements were carried out in doped poly(3-methylthiophene) devices by varying the carrier density. As the carrier concentration reduces the I-V characteristics indicate that the conduction mechanism is limited by metal-polymer interface, as also observed in impedance data. The temperature dependence of I-V in moderately doped samples shows a trap-controlled space-charge-limited conduction (SCLC); whereas in lightly doped devices injection-limited conduction is observed at lower bias and SCLC at higher voltages. The carrier density-dependent quasi-Fermi level adjustment and trap-limited transport could explain this variation in conduction mechanism. Capacitance measurements at lower frequencies and higher bias voltages show a sign change in values due to the significant variations in the relaxation behaviour for lightly and moderately doped samples. The electrical hysteresis increases as carrier density is reduced due to the time scales involved in the de-trapping of carriers.

Mapping of local electronic properties in nanostructured CMR thin films by Scanning Tunneling Microscopy (STM) and Local Conductance Map (LCMAP)

We have investigated the local electronic properties and the spatially resolved magnetoresistance of a nanostructured film of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature Scanning Tunneling Microscope (STM) operating in a magnetic field. The nanostructured thin films (thickness ≈500nm) of the CMR material La0.67Sr0.33MnO3 (LSMO) on quartz substrates were prepared using chemical solution deposition (CSD) process. The CSD grown films were imaged by both STM and atomic force microscopy (AFM). Due to the presence of a large number of grain boundaries (GB's), these films show low field magnetoresistance (LFMR) which increases at lower temperatures. The measurement of spatially resolved electronic properties reveal the extent of variation of the density of states (DOS) at and close to the Fermi level (EF) across the grain boundaries and its role in the electrical resistance of the GB. Measurement of the local conductance maps (LCMAP) as a function of magnetic field as well as temperature reveals that the LFMR occurs at the GB. While it was known that LFMR in CMR films originates from the GB, this is the first investigation that maps the local electronic properties at a GB in a magnetic field and traces the origin of LFMR at the GB.

Biaxial strain induced electrical inhomogenities and phase separation in the ferromagnetic metallic phase in thin films of La0.7Ca0.3MnO3: A scanning tunneling potentiometry/spectroscopy study

We have investigated the effect of biaxial strain on local electrical/electronic properties in thin films of La0.7Ca0.3MnO3 with varying degrees of biaxial strain in them. The local electrical properties were investigated as a function of temperature by scanning tunneling spectroscopy (STS) and scanning tunneling potentiometry (STP), along with the bulk probe like conductance fluctuations.The results indicate a positive correlation between the lattice mismatch biaxial strain and the local electrical/electronic inhomogenities observed in the strained sample. This is plausible since the crystal structure of the manganites interfere rather strongly with the magnetic/electronic degrees of freedom. Thus even a small imbalance (biaxial strain) can induce significant changes in the electrical properties of the system.

An Investigation into the Dependence of the Arc Voltage on the Parameters of the Short-Circuit Current and the Design of the Vacuum Interrupter

The vacuum interrupter is extensively employed in the medium voltage switchgear for the interruption of the short-circuit current. The voltage across the arc during current interruption is termed as the arc voltage. The nature and magnitude of this arc voltage is indicative of the performance of the contacts and the vacuum interrupter as a whole. Also, the arc voltage depends on the parameters like the magnitude of short-circuit current, the arcing time, the point of opening of the contacts, the geometry and area of the contacts and the type of magnetic field. This paper investigates the dependency of the arc voltage on some of these parameters. The paper also discusses the usefulness of the arc voltage in diagnosing the performance of the contacts.